Adenosine is a potent neuromodulator in brain having marked inhibitory effects on neuronal firing and the activity of adenyl cyclase. The recent availability of two metabolically stable adenosine analogues N6 cyclohexyl-(3H) adenosine, (3H) CHA, and (3H) diphenyl xanthine, (3H) DPX has made possible the direct analysis of adenosine receptors in brain. We have characterized the binding of these compounds in brain showing two binding sites for (3H) CHA with affinity constants of 0.7 and 2.4 nM and one binding site for (3H) DPX (kd = 6.2 nM). The metals copper and zinc have been shown to be highly potent inhibitors of (3H) CHA binding while both calcium and magnesium increase binding. Developmental studies show that (3H) CHA and (3H) DPX binding sites appear between 5 and 10 days after birth with cerebellar receptors developing sooner than those in the forebrain. A method has also been devised for the autoradiographic visualization of adenosine receptors and their distribution has been mapped. Especially high levels of receptor are seen in the superficial layer of the superior colliculus and the cerebellar cortex.